At its base level, a content management system (CMS) manages your content. However, that merely realigns it’s name.
Content management systems vary in complexity and price. At the low end you will find many free versions that do little more than provide a place for you to write webpages within a browser and give you a basic theme to display to your users.
High end content management systems will allow for multiple administrators with varying security roles. Sometimes these roles are site wide, other times they cover just a set of pages or categories. They will provide a way to upload and track media content (images, videos, audio clips, PDFs, etc) including providing versioning of the content so you can easily update all references to a file across the website.